MySQL 性能优化之 explain Extra 字段

创建表

create table s1 (
    id int auto_increment,
    key1 varchar(100),
    key2 int,
    key3 varchar(100),
    key_part1 varchar(100),
    key_part2 varchar(100),
    key_part3 varchar(100),
    common_field varchar(100),
    primary key(id),
    index idx_key1(key1),
    unique index idx_key2(key2),
    index idx_key3(key3),
    index idx_key_part(key_part1, key_part2, key_part3)
);

create table s2 (
    id int auto_increment,
    key1 varchar(100),
    key2 int,
    key3 varchar(100),
    key_part1 varchar(100),
    key_part2 varchar(100),
    key_part3 varchar(100),
    common_field varchar(100),
    primary key(id),
    index idx_key1(key1),
    unique index idx_key2(key2),
    index idx_key3(key3),
    index idx_key_part(key_part1, key_part2, key_part3)
);
insert into s1(id, key2) values(1,1);

NULL

主键或唯一二级索引等值查询时

mysql>  explain select * from s1 where id = 1;
+----+-------------+-------+------------+-------+---------------+---------+---------+-------+------+----------+-------+
| id | select_type | table | partitions | type  | possible_keys | key     | key_len | ref   | rows | filtered | Extra |
+----+-------------+-------+------------+-------+---------------+---------+---------+-------+------+----------+-------+
|  1 | SIMPLE      | s1    | NULL       | const | PRIMARY       | PRIMARY | 4       | const |    1 |   100.00 | NULL  |
+----+-------------+-------+------------+-------+---------------+---------+---------+-------+------+----------+-------+
1 row in set, 1 warning (0.00 sec)

二级索引等值查询时

mysql>  explain select * from s1 where key1 = 'abc';
+----+-------------+-------+------------+------+---------------+----------+---------+-------+------+----------+-------+
| id | select_type | table | partitions | type | possible_keys | key      | key_len | ref   | rows | filtered | Extra |
+----+-------------+-------+------------+------+---------------+----------+---------+-------+------+----------+-------+
|  1 | SIMPLE      | s1    | NULL       | ref  | idx_key1      | idx_key1 | 403     | const |    1 |   100.00 | NULL  |
+----+-------------+-------+------------+------+---------------+----------+---------+-------+------+----------+-------+
1 row in set, 1 warning (0.01 sec)

全表查询时

mysql>  explain select * from s1;
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------+
| id | select_type | table | partitions | type | possible_keys | key  | key_len | ref  | rows | filtered | Extra |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------+
|  1 | SIMPLE      | s1    | NULL       | ALL  | NULL          | NULL | NULL    | NULL |    2 |   100.00 | NULL  |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------+
1 row in set, 1 warning (0.00 sec)

Using index

使用覆盖索引

mysql>  explain select key1 from s1 where key1 = 'abc';
+----+-------------+-------+------------+------+---------------+----------+---------+-------+------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key      | key_len | ref   | rows | filtered | Extra       |
+----+-------------+-------+------------+------+---------------+----------+---------+-------+------+----------+-------------+
|  1 | SIMPLE      | s1    | NULL       | ref  | idx_key1      | idx_key1 | 403     | const |    1 |   100.00 | Using index |
+----+-------------+-------+------------+------+---------------+----------+---------+-------+------+----------+-------------+
1 row in set, 1 warning (0.00 sec)

Using index condition

索引条件下推

mysql>  explain select * from s1 where key_part1 = 'abc' and key_part2 like '%abc';
+----+-------------+-------+------------+------+---------------+--------------+---------+-------+------+----------+-----------------------+
| id | select_type | table | partitions | type | possible_keys | key          | key_len | ref   | rows | filtered | Extra                 |
+----+-------------+-------+------------+------+---------------+--------------+---------+-------+------+----------+-----------------------+
|  1 | SIMPLE      | s1    | NULL       | ref  | idx_key_part  | idx_key_part | 403     | const |    1 |    50.00 | Using index condition |
+----+-------------+-------+------------+------+---------------+--------------+---------+-------+------+----------+-----------------------+
1 row in set, 1 warning (0.00 sec)

Using where

使用全表扫描

mysql>  explain select * from s1 where common_field = 'abc';
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key  | key_len | ref  | rows | filtered | Extra       |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------------+
|  1 | SIMPLE      | s1    | NULL       | ALL  | NULL          | NULL | NULL    | NULL |    2 |    50.00 | Using where |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------------+
1 row in set, 1 warning (0.00 sec)

Using filesort

使用文件排序（没有索引可以利用）

mysql>  explain select * from s1 order by common_field;
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+----------------+
| id | select_type | table | partitions | type | possible_keys | key  | key_len | ref  | rows | filtered | Extra          |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+----------------+
|  1 | SIMPLE      | s1    | NULL       | ALL  | NULL          | NULL | NULL    | NULL |    2 |   100.00 | Using filesort |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+----------------+
1 row in set, 1 warning (0.00 sec)

Using join buffer (hash join)

连接查询时，没有索引可以利用时。MySQ:会分配一块名叫join buffer的内存块来加快查询速度。

也叫：基于块的嵌套循环算法。

mysql>  explain select * from s1 inner join s2 on s1.common_field = s2.common_field;
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+--------------------------------------------+
| id | select_type | table | partitions | type | possible_keys | key  | key_len | ref  | rows | filtered | Extra                                      |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+--------------------------------------------+
|  1 | SIMPLE      | s2    | NULL       | ALL  | NULL          | NULL | NULL    | NULL |    1 |   100.00 | NULL                                       |
|  1 | SIMPLE      | s1    | NULL       | ALL  | NULL          | NULL | NULL    | NULL |    2 |    50.00 | Using where; Using join buffer (hash join) |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+--------------------------------------------+
2 rows in set, 1 warning (0.00 sec)

Using temporary

使用临时表（去重、排序）。

在执行一些distinct、group by、union等语句时，如果不能利用索引来完成，MySQL会通过临时表来执行。

mysql>  explain select distinct common_field from s1;
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-----------------+
| id | select_type | table | partitions | type | possible_keys | key  | key_len | ref  | rows | filtered | Extra           |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-----------------+
|  1 | SIMPLE      | s1    | NULL       | ALL  | NULL          | NULL | NULL    | NULL |    2 |   100.00 | Using temporary |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-----------------+
1 row in set, 1 warning (0.00 sec)

No tables used

mysql>  explain select 1;
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+----------------+
| id | select_type | table | partitions | type | possible_keys | key  | key_len | ref  | rows | filtered | Extra          |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+----------------+
|  1 | SIMPLE      | NULL  | NULL       | NULL | NULL          | NULL | NULL    | NULL | NULL |     NULL | No tables used |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+----------------+
1 row in set, 1 warning (0.00 sec)

Impossible WHERE

mysql>  explain select * from s1 where 1 != 1;
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+------------------+
| id | select_type | table | partitions | type | possible_keys | key  | key_len | ref  | rows | filtered | Extra            |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+------------------+
|  1 | SIMPLE      | NULL  | NULL       | NULL | NULL          | NULL | NULL    | NULL | NULL |     NULL | Impossible WHERE |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+------------------+
1 row in set, 1 warning (0.00 sec)

Zero limit

一条记录也不查看

mysql>  explain select * from s1 limit 0;
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+------------+
| id | select_type | table | partitions | type | possible_keys | key  | key_len | ref  | rows | filtered | Extra      |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+------------+
|  1 | SIMPLE      | NULL  | NULL       | NULL | NULL          | NULL | NULL    | NULL | NULL |     NULL | Zero limit |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+------------+
1 row in set, 1 warning (0.00 sec)